1. Field of the Invention
The invention relates to variable gain amplifiers, and in particular to variable gain amplifiers with controllable gain gradient over temperature.
2. Description of the Related Art
Many communication applications require variable gain exponentially proportional to an input voltage. Since, on a dB scale, a gain curve becomes a straight line, this is commonly referred to as “linear-in-dB”. For example, linear-in-dB variable gain amplifiers are used in transceivers for cellular phones. A variable gain amplifier (VGA) serves as an output power controller in the automatic gain control loop of the transmitter to regulate the power of the signal transmitted from the cellular phone. A variable gain amplifier is also used in the receiver to vary the amount of gain according to the specific receiver operation and the strength of the received signal, to maintain a constant signal level.
FIG. 1 shows a conventional variable gain amplifier. In transmitters and receivers in conventional communication application, the gain of VGAs is controlled by a control signal SC which is temperature independent to prevent temperature variation. FIG. 2 shows the relationship between the control signal SC and gain (in dB scale) of the variable gain amplifier, with gain gradient of the conventional variable gain amplifier is constant over temperature. The gain (G) can be represented by the following formula:
                    G        =                                            S              i                                      S              o                                =                                    G              max                        +                                          K                G                            ⁡                              (                                                      S                    C                                    -                                      S                    offset                                                  )                                                                        (        1        )            
Wherein So represents output signal, Si represents input signal, SC represents gain control signal, KG represents gain slope, Soffset represents gain control signal at maximum gain and Gmax represents maximum gain.